In the core of a nuclear reactor, the rods or "pencils" of combustible material are generally regrouped in parallel in bundles or assemblies comprising end members and intermediate supporting grids maintaining the spacing of the rods. Supporting tubes or guide tubes directly connect the end members, in places taking the place of rods in the intermediate supporting grids.
The operation of the reactor involves the possibility of replacing or checking certain rods, which makes it necessary to dismantle the assemblies, an operation which is carried out under immersion in a great depth of water for protection against radiation.
The problem therefore consists in enabling the connection of the end members to the guide tubes to be sufficiently easy to dismantle to be operable at a distance and under immersion, and at the same time to be sufficiently reliable to avoid any risk of release of the connection through untimely unscrewing, and above all any risk of having the connecting members separate from the assembly and being entrained in the circuit of the primary cooling fluid of the reactor.
A solution has already been proposed in applicant's French Pat. No. 76-31554, published under No. 2.368.785, and describing a screw connection which can therefore be easily maneuvered at a distance, with a device for locking in rotation by deformation of a thin element of the screw head to engage it in a recess in the end member, an operation which is likewise easy to carry out at a distance. This arrangement does in fact render it possible to prevent the accidental unscrewing of the screw and its loss in the primary circuit. Nevertheless, it leaves a risk in the case of an accidental fracture of the screw, particularly if this fracture takes place in the relatively more fragile connecting region between the head and the threaded shank. In such case, in fact, the head would become entirely free and could therefore leave its seating and become an uncontrolled foreign body in the core of the reactor and in the primary circuit. This applies, in particular, to the case of the screws connecting the guide tubes to the lower end members when the assembly is in position in the core.